Processing of nuclear magnetic resonance signals is an important aspect of hydrocarbon recovery. In order to determine if hydrocarbons are located within a geological stratum, drilling operators place a nuclear magnetic resonance tool in a drill string. Once activated, the nuclear magnetic resonance tool emits a signal or set of signals that penetrate the geological stratum and reflect off different features back to the tool. These signals are subsequently processed to determine if there is a presence of hydrocarbons.
The most common nuclear magnetic resonance data processing methods use so called phase alternating pairs. This method was developed to removing certain noises or “ringing” so that an overall better analysis of the returned signals could be accomplished. In general, conventional methods to evaluate the returned signals seek to increase the signal to noise ratio. Larger and more defined signal components compared with noise, produces more accurate results. Such conventional systems are acquired as weighted sums of digitized nuclear magnetic resonance signals centered around an anticipated echo peak maximum.
In some situations, the actual echo peaks shift compared to the anticipated echo peak maximums. The “window” for the actual signal retrieval will not coincide with the anticipated signal retrieval window. This results in lost signals and improper processing. Conventional systems, therefore leave much to be desired under such conditions and have significant limitations.
Conventional systems undertake certain measures to minimize the error that can be caused by noise. One such method that conventional systems utilize is acquiring successive echo trains for nuclear magnetic resonance signals with alternating phase. The noise, sometimes referred to as antenna ringing, however, is created with constant phase, thus the noise can be identified and eliminated from the returned signal. Thus, the noise generated is not entirely random. This is achieved by alternating the phase of the ninety (90) degree excitation pulse while keeping the phase of subsequent refocusing pulses constant for all trains.
While the above-described method works well for identifying the noise created with constant phase, the conventional systems have a significant limitation. If the noise is not of a constant phase or changes over time, the alternating phase method is less effective, as that method works best with noise that is created with a constant phase.
Other conventional systems estimate a ringing signal by adding specifically designed auxiliary measurements to the nuclear magnetic resonance sequence. Still other conventional systems propose a method to estimate ringing amplitudes either from successive alternating phase Car-Purcell-Meiboom-Gill trains or from auxiliary sequences.
All of the above methodologies apply to amplitudes and amplitude measurement, whether referring to NMR echoes or ringing signals. Ringing filtering and noise reduction are both achieved by combining multiple (at least two) measurements of different phase or frequency.
None of the methods described evaluate or utilize the actual echo shapes received from the reflected signals, instead they rely on assumptions of what will be received and the times that they will be received.